JP-2008-101626A shows an internal combustion engine provided with an exhaust gas recirculation (EGR) system. In the EGR system, a part of exhaust gas emitted from the engine is recirculated into an intake passage downstream of a throttle valve through an EGR passage. The quantity of the recirculated exhaust gas is controlled by an EGR valve. In this EGR system, the quantity of exhaust gas (EGR-quantity) passing through the EGR valve is computed by means of an EGR valve model which simulates a behavior of the recirculated exhaust gas passing through the EGR valve. When the engine is stably running, it is deemed that the quantity of the exhaust gas passing through the EGR valve is equal to the quantity of the exhaust gas flowing into a cylinder. When the engine is in a transitional running condition, the quantity of exhaust gas (EGR-quantity) flowing into the cylinder is computed based on the quantity of the exhaust gas passing through the EGR valve by means of a EGR diffusion model (time delay+first order lag) which simulates a behavior of the exhaust gas flowing in an intake pipe toward an intake port of the engine.
In order to improve fuel economy of the engine equipped with an EGR system, it is necessary to control the quantity of the recirculated exhaust gas flowing into a cylinder so that gas including exhaust gas is efficiently combusted in a cylinder. Thus, it is necessary to estimate the quantity recirculated exhaust gas flowing into the cylinder with high accuracy.
The EGR system includes two types of system. In one system, the EGR gas is recirculated into an intake passage downstream of the throttle valve. In the other system, the EGR gas is recirculated into an intake passage upstream of the throttle valve. In this system where the EGR gas is recirculated into an intake passage upstream of the throttle valve, since an inner volume of the intake passage is relatively large, a variation in EGR-quantity is slow relative to a variation in the EGR valve opening.
Even if an estimation method of the EGR-quantity shown in JP-2008-101626A is applied to the system where the EGR gas is recirculated into an intake passage upstream of the throttle valve, it is difficult to accurately estimate the EGR-quantity.